Nuclear Instruments and Methods in Physics Research A 440 (2000) 618}625 Observation of a triple correlation in ternary "ssion: is time reversal invariance violated? P. Jesinger*, A. Ko K tzle, A.M. Gagarski, F. Go K nnenwein, G. Danilyan, V.S. Pavlov, V.B. Chvatchkin, M. Mutterer, S.R. Neumaier, G.A. Petrov, V.I. Petrova, V. Nesvizhevsky, O. Zimmer, P. Geltenbort, K. Schmidt, K. Korobkina Physikalisches Institut, Auf der Morgenstelle 14, 72076 Tu ( bingen, Germany PNPI of RAS, Gatchina, Leningrad District 188350, Russia ITEP, Moscow 117259, Russia Institut fu ( r Kernphysik, TU Darmstadt, 64289 Darmstadt, Germany ILL Grenoble, Avenue des Martyrs, BP156, 38042 Grenoble, France Kurchatov Institute, Moscow 123182, Russia Abstract In ternary "ssion, besides the two main "ssion fragments, a third (usually light) charged particle is emitted. A triple correlation has been studied involving the momenta for a speci"c "ssion fragment p (  , the momenta of the ternary particle p (  and the spin of the polarized cold neutron inducing "ssion  L . The correlation observable B" L ) [p (  p (  ] reverses sign upon time reversal and thus a non-vanishing value for the expectation value B could possibly be due to TRI being violated. However, "nal-state interactions or speci"c properties of the emission mechanism for ternary particles could equally well lead to a non-zero B with TRI being perfectly conserved. The reaction chosen was U(n,f). An unexpectedly large correlation was observed. From the raw data the value for B is B"!(0.78$0.02)10 with the sign corresponding to light fragments. Corrections for neutron polarization, geometric e$ciency, resolution of detectors and background increase this "gure by a factor of (1.5$0.3).  2000 Elsevier Science B.V. All rights reserved. Keywords: Ternary "ssion; Time reversal invariance 1. Introduction Ternary "ssion is a rare decay mode of nuclear "ssion (branching ratio +0.3%), where three * Corresponding author.  Supported by the BMBF, Bonn, under contract number 06Tu ( 669. charged particles emerge from the decay of a heavy nucleus, viz. two main "ssion fragments and a lighter charged particle. In about 90% of all ternary events, the light particle is an -nucleus, in 7% a triton is emitted. The preferred direction for the emission is in a plane perpendicular to the axis of the two main "ssion fragments. The present experiment originally was proposed to be a test for Time Reversal Invariance (TRI) making use of the 0168-9002/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 5 1 - 7